The present invention relates to a method and apparatus for capping upstanding stems to form mechanical fastener hooks. More particularly the invention relates to a method for forming asymmetrical capped hook shapes with greater uniformity and manufacturability.
Hook and loop mechanical fasteners are widely used for a vast array of products and applications. The methods for forming hook materials for use in these hook and loop type mechanical fasteners are widely varied. Over the years it has been found that hooks have different engagement mechanisms and characteristics with mating loop materials, depending on the hooks and/or the loop. This limits the practical applicability of any particular hook forming technique to producing hooks applicable for only certain types of uses or for use with only certain types of mated loop materials.
Some of the earliest hook materials were formed using the processes described in U.S. Pat. Nos. 2,717,437 and 3,009,235 in which special warps of upstanding nylon thread are cut so as to form open ended nylon hooks and nonfunctional upstanding stems. Hooks formed by these types of methods are large (e.g., about 2 mm) requiring the use of large open pile loop materials and have a rather low number of hooks per unit area. These hooks are also quite abrasive and therefore not well suited for uses where the hooks would likely contact sensitive skin. Hooks of this type are still in use today due to their durability in long term use. Similar types of hook structures are formed in accordance with the method of U.S. Pat. No. 3,594,865 in which a thermoplastic material is directly formed into a J-shaped "wire" hook by use of shallow J-shaped dies. These "wire dies" are formed into a continuous loop of mold material which mold passes through an extruder. The extruder forces molten plastic, such as nylon, into the wire dies while also impregnating a fabric web immediately beneath the wire die mold material. On exiting the extruder, excess thermoplastic resin is stripped from the surface of the wire die molds. The resilient hooks remain on the backing when the die is removed by essentially just pulling the now solidified hooks and backing away from the wire die mold material. U.S. Pat. No. 3,594,863 relates to a similar apparatus for producing a hook bearing strip. Both these patents state that the methods described can produce a wide variety of shapes. U.S. Pat. No. 3,594,865 states that the traditional method for directly injection molding a hook is limited to shapes which must taper from base to tip. However, the hook shapes formed by these patents are relatively large and the hooks must taper from the outer face to the opposing face along the length of the hook.
The traditional molded type of hook shapes referred to in U.S. Pat. No. 3,594,865 are like those described in U.S. Pat. Nos. 4,984,339 and 5,315,740. These patents disclose molded J-shaped hooks which have a profile defined by an inner smooth contoured, generally concave face and a generally convex shaped outer face. The hook tapers smoothly and continuously in width from the hook base to the hook free end. The hook allegedly is designed so that it will not deform to release a loop engaged with the hook in shear mode or at a desired applied force. The later patent discloses a similar hook which has a low displacement volume for the area defining the hook tip. This is described as desirable for use in applications such as disposable diapers and the like. Although these J-shaped hooks are generally adequately performing materials, they are extremely difficult to manufacture, particularly when very small hooks are made such as described in the '740 patent. Small complex shaped mold cavities are extremely difficult to manufacture and when forming extremely small hooks a proportionately larger number of J-shaped hook mold cavities must be formed. Small complex-shaped mold cavities are also much more susceptible to clogging and loss of mold cavity definition due to wear.
Extremely flexible and low cost methods for forming hooks of a wide variety of sizes and shapes are described in PCT Application Nos. WO 94/23610 and 92/04839 as well as U.S. Ser. No. 08/723,632. Using methods described in these patents and patent applications, a backing having a large number of upstanding thermoplastic stems is fed through a gap between a nip formed by, for example, two calender rolls. The upper nip is smooth and heated so that the distal ends or tips of the stems are deformed under heat and mechanical pressure, forming various types of cap structures depending on the nip conditions selected, the relative speed of the stems in the nip, and the size and shape of the stems. The undeformed stem portion and the formed cap together form a hook structure. The precursor material, a backing having upstanding undeformed thermoplastic stems, can be formed by molding techniques. However, the formation and use of mold cavities in the shape of upstanding stems is much simpler and less problematic than forming J-shaped hook molds. For example, these simple mold cavity shapes are much less likely to clog or adversely wear, depending on the selection of the mold materials. Further, it has been found that by using this method it is relatively easy to form large numbers of small closely spaced hooks per unit area, which is particularly desirable for engagement with relatively low loft woven or nonwoven loop materials. These low loft loop materials are generally also low cost making this hook structure extremely desirable for low cost limited use applications such as disposable garments. Also advantageous is the tactile feel of these hook materials. Due to the high density of hooks and/or the hooks having a relatively flat or planar upper surface, the hooks are extremely skin friendly, non-abrasive and have a film-like texture. The preferred hook materials are essentially not even discernible as having hooks with casual skin contact. This also makes the hooks useful for disposable garments used close to the skin (e.g., diapers or surgical gowns). The present invention is directed at a method for improving the above method of hook formation and the hooks formed thereby.